1. Field of the Invention
The present invention relates to a surface acoustic wave filter including a plurality of one-port surface acoustic wave resonators (one-port SAW resonators) and, more particularly, to a surface acoustic wave filter having a ladder-type circuit configuration including a plurality of one-port SAW resonators connected to each other in a ladder-type configuration.
2. Description of the Related Art
A conventional band filter includes a surface acoustic wave filter having a ladder type circuit configuration of a plurality of one-port SAW resonators connected to each other in a ladder type configuration (for example, Japanese Examined Patent Publication No. 56-19765).
The circuit of a surface acoustic wave filter of this type is partially shown in FIG. 10, a series arm is provided between an input terminal 51 and an output terminal (not shown), and a series resonator S1 is included in the series arm. A parallel resonator P1 is connected between the series arm and a reference potential, and thereby, a parallel arm is formed. Although only one series resonator S1 and one parallel resonator P1 are shown in FIG. 10, a plurality of series arms containing a plurality of series resonators and a plurality of parallel arms containing parallel resonators are arranged to extend in the direction from the input terminal 51 toward the output terminal.
Ordinarily, the one-port SAW resonators each defining the above-mentioned series resonator S1 and the parallel resonator P1 have an electrode structure shown in FIG. 11. As seen in FIG. 11, the one-port SAW resonator has a structure in which one pair of interdigital electrodes 52a and 52b are provided on a piezoelectric substrate (not shown) whereby one interdigital transducer (IDT) 52 is produced. Grating type reflectors 53 and 54 are arranged on the opposite sides of the IDT 52 in the surface acoustic wave propagation direction.
FIG. 12 illustrates the typical filter characteristics of a surface acoustic wave device having a ladder type circuit configuration containing the above-described one-port SAW resonators defining the series resonator and the parallel resonator.
For a band filter, it is necessary to increase the attenuation in the frequency range outside of the filter pass-band. Accordingly, for the purpose of increasing the attenuation in the frequency range outside of the filter pass band, a mirror image type connection structure shown in FIG. 13 is generally used.
A mirror image type connection structure is a structure in which a connection structure including one series resonator and one parallel resonator is connected to a connection structure including one series resonator and one parallel resonator such that both connection structures define a mirror image of each other relative to the boundary between the connection structures. More particularly, the series and parallel resonators of the connection structure shown by a broken line A in FIG. 13 are connected to portions of the connection structure surrounded by a broken line B adjacent to the broken line A such that both connection structures have a mirror image relationship relative to a boundary therebetween. Similarly, the connection structure surrounded by a broken line C is arranged to have a mirror image relationship with the connection structure surrounded by the broken line B relative to a boundary therebetween.
More specifically, on the opposite sides of the boundary, the parallel resonators P1 and P2 are arranged in the boundary area of the connection structures surrounded by the broken lines A and B, and the series resonators S1 and S2 are arranged so as to be spaced far from the joining area of both of the connection structures, respectively.
For a surface acoustic wave filter of the above described type, it is required that its reflection characteristic (VSWR) is low in the filter pass band. It is known that in order to reduce the reflection characteristic to a value of about 2.0, which is generally required, the difference between the resonant frequency of the series resonators S1, S2, and S3 and that of the parallel resonators P1, P2, and P3 is adjusted. See, for example, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS TECHNICAL REPORT, JAPAN US95-25, EMD95-21, 33 (1995-07), p39-p46.
More particularly, it is known that in the surface acoustic wave filter having a ladder type circuit configuration shown in FIG. 13, VSWR can be reduced by changing the difference between the resonant frequency of the series resonators S1, S2, and S3 and that of the parallel resonators P1, P2, and P3.
In the above-described prior art, it is described that VSWR is reduced in a filter pass band by changing the difference between the resonant frequency of the series resonators S1, S2, and S3 and that of the parallel resonators P1, P2, and P3. In this prior art, it is described that VSWR is varied as shown in FIG. 14 when the above resonant frequency difference is changed.
More particularly, in the case that the difference between the resonant frequency of the series resonator and that of the parallel resonator is small, VSWR is large on the lower frequency side, as shown by an arrow C in FIG. 14. On the contrary, in the case that the above frequency difference is small, VSWR is large on the higher frequency side as shown by an arrow D in FIG. 14.
Thus, there is a problem with the prior art described above in that if VSWR on the lower frequency band side is reduced, VSWR is increased and degraded on the higher frequency band side.